Question

Which of the following is the most precise device for measuring length?

• A. A vernier callipers with 20 divisions on the sliding scale.
• B. A screw gauge of pitch 1 mm and 100 divisions on the circular scale.
• C. An optical instrument that can measure length to within a wavelength of light.
  
• D. All instruments have same precision.
  

Answer 1

The correct option is C An optical instrument that can measure length to within a wavelength of light.

Most precise device is the one with the minimum least count
1. Vernier Calliper
20 divisions on the sliding scale
$L.C=\frac{1}{20}mm=0.05mm$
2. Screw Gauge
pitch 1 mm and 100 divisions on the circular scale
$L.C=\frac{1mm}{100}=0.01mm$
3.optical instrument
within a wavelength of light
Wavelenght of light $\approx$ $589nm$
Hence Optical instrument is the most precise